BOSTON (Reuters) - Doctors who waited just 75 seconds after the final heartbeat before removing the hearts of dying newborns for transplants said on Wednesday they improved their odds of success but have also raised ethical questions about organ harvesting. The cases of two children who died between May 2004 and May 2007, and a third in which doctors waited three minutes, are detailed in a report and a series of commentaries in the New England Journal of Medicine.